1. Field of the Invention
This invention relates to improvements in methods and circuitry for driving polyphase dc motors, and, more particularly, to improvements in methods and circuitry for driving polyphase dc motors that employ pulse width modulation (PWM) chopping, and still more particularly to improved methods and circuitry for driving polyphase dc motors that provide or enable reliable zero crossing determination for commutation in the presence of a PWM chopping signal.
2. Description of the Prior Art
Although the present invention pertains to polyphase dc motors, in general, it finds particular application in conjunction with three phase dc motors, particularly of the brushless, sensorless type which are used for rotating data media, such as found in computer related applications, including hard disk drives, CD ROM drives, floppy disks, and the like. In computer applications, three phase brushless, sensorless dc motors are becoming more popular, due to their reliability, low weight, and accuracy.
Motors of this type can typically be thought of as having a stator with three coils connected in a "Y" configuration, although actually, a larger number of stator coils are usually employed with multiple motor poles. Typically, in such applications, eight pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three "Y" connected coils, connected in three sets of four coils, each physically separated by 90.degree..
In operation, the coils are energized in sequences. In each sequence, a current path is established through two coils of the "Y", with the third coil left floating. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. Moreover, the sequence is defined such that when the floating coil is switched into the current path, current will flow in the same direction in the coil which was included in the prior current path. In this manner, six commutation sequences are defined for each electrical cycle in a three phase motor.
In the past, during the operation of a such polyphase dc motor, it has been recognized that maintaining a known position of the rotor is an important concern. There have been various ways by which this was implemented. The most widely used way, for example, was to start the motor in a known position, then develop information related to the instantaneous or current position of the rotor. One source of such instantaneous position information was developed as a part of the commutation process, and involved identifying the floating coil, and monitoring its back emf, that is, the emf induced into the coil as it moves through the magnetic field provided by the stator.
When the voltage of the floating coil crossed a particular reference voltage, often zero, the position of the rotor was assumed to be known. (Herein, the crossing of the zero voltage, or the reference voltage, is referred to as "a zero crossing".) When a zero crossing occurred, the rotor coil commutation sequence was incremented to the next phase, and the process repeated.
In addition, frequently polyphase dc motors employ pulse width modulation (PWM) to modulate the drive current to the coils of the motor. PWM techniques are often used to reduce the power dissipation in polyphase motors by chopping the current in the coils of the motor at their peak current levels, to achieve maximum torque, to allow rapid accelerations, and to reduce the power dissipated in the chip to a level proportional to the duty cycle.
Typically when the motor is operating, one or more of the coils is floating (or at high impedance) in each commutation state, with the remaining coils being actively driven to produce the desired rotation of the rotor of the motor. When, however, the PWM mode is entered, the drive current is temporarily interrupted and allowed to decay at a predesigned rate. Generally an RC discharge network is provided to determine the decay of the current within the drive coils. When the voltage across the resistor sensing the total current through the motors has decayed to a predetermined level, the drive current is then reapplied to the active coils of the motor. This is known in the art as "PWM chopping".
The PWM chopping of the active coils generates noise inductively coupled to the floating coil or coils of the motor. It will be recalled that the back emf of the floating coils is monitored to determine the zero voltage crossing for the purpose of sensorless position detection of the rotor of the motor. However, due to the inductively coupled noise from the back emf chopping signal into the floating coil, the actual or true zero crossing may be difficult to properly detect, since the voltage levels may actually appear higher or lower than the levels which would normally appear in the absence of the PWM chopping signal. Thus, in the presence of PWM chopping, what is needed is a method and circuit for minimizing the effect of the PWM chopping signal to enable the zero crossings to be accurately detected for accurate rotor position determination.